These Two Ancient Fossilized Ancestors of Mammals Reveal Early Evolutionary Diversification

Two newly discovered fossil mammals may tell scientists a bit more about the creatures that existed 160 million years ago. The new fossils reveal that mammalian diversity really began around that time period.

One of the fossils is of the tree-climbing Agilodocodon scansorius, which is one of the earliest-known tree-dwelling ancestors of modern mammals. It had teeth adapted for a tree sap diet and was relatively small. The other fossil is of Docofossor brachydactylus. This is one of the earliest-known subterranean mammaliaforms, and possessed multiple adaptations similar to African golden moles. It had shovel-like paws and had distinct skeletal features that resemble patterns shaped by genes identified in living mammals.

"We consistently find with every new fossil that the earliest mammals were just as diverse in both feeding and locomotor adaptations as modern mammals," said Zhe-Xi Luo, one of the researchers, in a news release. "The groundwork for mammalian success today appears to have been laid long ago."

These new fossils reveal that both tree-dwelling and subterranean mammal ancestors evolved early. Both about the size of a shrew, these mammal ancestors had unique adaptations that were tailored to their respective ecological habitats.

"The finger and limb bone dimensions of Agilodocodon match up with those of modern tree-dwellers, and its incisors are evidence it fed on plant sap," said David Grossnickle, co-author of the new study. "It's amazing that these arboreal adaptions occurred so early in the history of mammals and shows that at least some extinct mammalian relatives exploited evolutionarily significant herbivorous niches, long before true mammals."

The findings reveal a bit more about how mammals diversified. More significantly, it shows that this process occurred early.

"We know that modern mammals are spectacularly diverse, but it was unknown whether early mammals managed to diversify in the same way," said Luo. "These new fossils help demonstrate that early mammals did indeed have a wide range of ecological diversity. It appears dinosaurs did not dominate the Mesozoic landscape as much as previously thought."